Retinex improves visual rendering of an image when lighting conditions
are not good. While our eye can see colors correctly when light is low,
cameras and video cams can't manage this well. The MSRCR (MultiScale
Retinex with Color Restoration) algorithm, which is at the root of the
Retinex filter, is inspired by the eye biological mechanisms to adapt
itself to these conditions. Retinex stands for Retina + cortex.
Besides digital photography, Retinex algorithm is used to make the
information in astronomical photos visible and detect, in medicine,
poorly visible structures in X-rays or scanners.
These options call for notions that only mathematicians and imagery
engineers can understand. In actual practice, the user has to grope
about for the best setting. However, the following explanations should
help out the experimented GIMP user.
-
Level
-
Here is what the plug-in author writes on his site
[PLUGIN-RETINEX]:
“To characterize color variations and the lightor, we make
a difference between (gaussian) filters responses at different
scales. These parameters allow to specify how to allocate scale
values between min scale (sigma 2.0) and max (sigma equal to the
image size)”...
-
Uniform
-
Uniform tends to treat both low and high intensity areas
fairly.
-
Low
-
As a rule of thumb, low does “flare up”
the lower intensity areas on the image.
-
High
-
High tends to “bury” the lower intensity
areas in favor of a better rendering of the clearer areas
of the image.
-
Scale
-
Determines the depth of the Retinex scale. Minimum value is 16, a
value providing gross, unrefined filtering. Maximum value is 250.
Optimal and default value is 240.
-
Scale division
-
Determines the number of iterations in the multiscale Retinex
filter. The minimum required, and the recommended value is three.
Only one or two scale divisions removes the multiscale aspect and
falls back to a single scale Retinex filtering. A value that is
too high tends to introduce noise in the picture.
-
Dynamic
-
As the MSR algorithm tends to make the image lighter, this slider
allows you to adjust color saturation contamination around the
new average color. A higher value means less color saturation.
This is definitely the parameter you want to tweak for optimal
results, because its effect is extremely image-dependent.
